Sol gel aluminous abrasives, have demonstrated substantial advantages over other premium abrasives in broad areas of coated abrasive applications since their introduction some few years ago. Such abrasives are generally made by drying and sintering a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO.sub.2. The dried material is crushed either before or after sintering to obtain irregular polycrystalline abrasive grits in a desired size range. The grits may be later incorporated in abrasive products such as coated abrasive disks or belts.
U.S. Pat. No. 4,314,827 to Leitheiser et al, discloses abrasive grits made by such a method in which the sintered grits contain irregular "snowflake" shaped alpha Al.sub.2 O.sub.3 crystals which are on the order of 5 to 10 microns in diameter. The spaces between the arms of a "snowflake" and between adjacent "snowflakes" are occupied by other phases such as a finely crystalline alumina magnesia spinel.
U.S. Pat. No. 4,623,364, which issued on Nov. 18, 1986 assigned to Norton Company, the assignee of this application, discloses a sol gel method for the manufacture of aluminous abrasive grits, and products other than abrasive grits such as coatings, thin films, fibers, rods or small shaped parts, having enhanced properties. In that patent the conversion of the hydrated alumina to alpha alumina is facilitated by the introduction of seed material into the gel or the gel precursor prior to drying. This can be accomplished by either wet vibratory milling of the gel or gel precursor with alpha alumina media, or by the direct addition of very fine seed particles in powder or other form. The alpha alumina so obtained (often herein called SG grain) has a very fine, uniform crystallite structure with substantially all the crystallites less than about 1 micron in size. Slightly larger sizes can be obtained by longer firing but this is not in general desirable. To make abrasive grits the seeded gel is dried, crushed and fired. The abrasive grits so produced may be used in the manufacture of products such as coated abrasive disks and grinding wheels. Alternatively, to make shaped parts or rods, the material may be formed or molded as by extrusion before firing. In the case of extrusion of rods, the rods are later cut or broken into appropriate lengths.
The present invention provides a coated abrasive with a long useful life and which require less applied power for a given cutting rate using a special form of abrasive grain.
The coated abrasives of the invention are freer cutting and generate less heat during grinding.
The coated abrasives of the invention are also found to cut at a more even rate and impart a more consistent surface finish to the work piece over an unusually long useful life.